Poly(9,9-disubstituted-fluorene-2,7-diyls) (polyfluorenes) exhibit the optical and electronic properties of inorganic semiconductors because of the presence of delocalized electrons in the π-orbital system. These polymers are especially desirable because the backbone is resistant to chemical and photochemical degradation and the fluorene structural units are locked into a planar-like configuration. Moreover, substituents at the C9 position of fluorene may be chosen to modify physical, chemical, and electronic properties without introducing torsional strain between adjacent fluorene structural units that would otherwise be disruptive to delocalization of the π-orbital system. For example, poly(9,9-di-n-octylfluorene-2,7-diyl), described in U.S. Pat. No. 5,708,130 by Woo et al. has been shown to by Grice, et al. (Applied Physics Letters, Vol. 73, 1998, p. 629–631) be an effective emitter for a blue light emitting diode (LED) and by Redecker, et al. (Applied Physics Letters, Vol. 73, 1998, p. 1565–1567) to exhibit advantageously high carrier mobility.
Methods to further modify optical and electronic properties of polyfluorenes through copolymerization of 9,9-disubstituted-fluorenes and various comonomers containing delocalized π-system electrons are described in U.S. Pat. Nos. 5,777,070, 5,708,130, and 6,353,083. An example of a particularly desirable comonomer is 4,7-bis(5-bromothien-2-yl)-2,1,3-benzothiadiazole, described in WO 00/46321, and having the following chemical structure:

The 4,7-bis(5-bromothien-2-yl)-2,1,3-benzothiadiazole is traditionally prepared from 4,7-bis(thien-2-yl)-2,1,3-benzothiadiazole, which in turn is prepared by the Stille coupling reaction of 4,7-dibromo-2,1,3-benzothiadiazole with tributyl(thien-2-yl)stannane as described by Kitamura et al., Chem. Mater., Vol. 8, 1996, pp. 570–578. Unfortunately, tributyl(thien-2-yl) stannane is a highly toxic and costly material that produces a toxic by-product tributyltin bromide. The process is further disadvantaged because the subsequent bromination reaction is carried out in a mixture of N-bromosuccinimide, chloroform, and acetic acid, which requires numerous crystallizations of the product to obtain a suitably pure material in the subsequent copolymerization reaction.
Accordingly, it would be advantageous to have a method of preparing 4,7-bis(5-bromothien-2-yl)-2,1,3-benzothiadiazole more safely and efficiently than methods described in the art.